/Engine/Plugins/Editor/UVEditor/Source/UVEditorTools/Public/Selection/UVToolSelectionAPI.h
Note: you should call with bForceRecalculate = true whenever calling this from OnCanonicalModified, because it is not guaranteed that the selection api will detect the mesh change (and therefore invalidate the cached value) before your call site, due to the order of OnCanonicalModified broadcasts
FVector3d GetUnwrapSelectionBoundingBoxCenter ( bool bForceRecalculate )